Syndepositional glauconite as a paleoenvironmental proxy - the lower Cenomanian Chalk of Cap Blanc Nez (N-France)

At Cap Blanc Nez (Channel coast, France), the chalk of Lower Cenomanian is very rich in glauconite. Glauconite authigenic origin and requires mobilization chemical elements for its growth: Si, Fe K. If we already know thanks to elementary geochemistry (Ge/Si ratio) that silica flint present originates from dissolution sponges, it same glauconite? This question only makes sense if glauconite proved be autochthonous synsedimentary, not reworked during transgression. In addition, wanted whether study content trace could provide information on conditions authigenesis glauconious chalk. The clay has been examined green minerals were extracted rock their morphology, mineralogy, (major & elements) grain size. consists calcium carbonate, smectite true glauconite, form pellets, sometimes with rare traces quartz some centimeter-scale phosphate gravel. grain-size distribution glauconites varies one sample another always poorly sorted, which militates favor an (not reworked) these minerals. also suggested by virtual absence terrigenous minerals, except known potential wide marine environment. samples shows a homogeneous composition major elements, K2O greater than 8%. characterizes as being evolved, indicates long formation time (> 100 ky) therefore extremely reduced or irregular sedimentation rate. Here, are germanium, impossible identify source (unlike what possible flints). It cannot said results sponges but this enrichment Ge, coupled vanadium molybdenum, slightly reducing deposition milieu (suboxic). Such usually organic matter accumulation, here, due protracted hiatuses. Lastly, trapped relatively large amounts Ge exposure seawater, chronometer assessing duration authigenesis, compartment cycle germanium.